A fascinating phenomenon with a forbidding name (parametric down conversion) occurs in certain crystals. A single photon enters, but two emerge. There are myriad pair combinations; but in each case, the sum of the total energy and momentum of the two output photons exactly equals those of the single input photon.
Parametric down conversion is finding a host of uses in physics, from testing the efficiency of high-tech single-photon detectors to cutting-edge experiments in quantum mechanics.
Alan Migdall of the National Institute of Standards and Technology explains and demonstrates how it's done.